Neutral-gas-phase metal abundances in Zn II-selected quasar absorption line systems near redshift z = 1.2

Abstract

We present 10 metallicity measurements for quasar absorbers near z = 1.2 that were selected for having unusually significant Zn II absorption in their Sloan Digital Sky Survey (SDSS) spectra. Follow-up UV space spectroscopy of the Lyα region shows that all 10 have damped Lyα (DLA) absorption, corresponding to neutral hydrogen column densities in the range 2.4 × 10²⁰ ≤ N(H I) ≤2.5 × 10²¹ atoms cm^-2 and indicating that the gas is very optically thick and essentially neutral. The sample is a very small subset of systems compiled by searching the University of Pittsburgh catalogue of ≈30 000 intervening Mg II absorption line systems in SDSS quasar spectra up to DR7. We started by isolating ≈3000 that had strong Mg II absorption in the redshift interval 1.0 < z < 1.5 in brighter background quasars. Of these, 36 exhibited significant absorption near Zn II. Space UV spectroscopy was then obtained for nine of these (25 per cent of the total), and a tenth system in a fainter quasar was found in the Hubble Space Telescope archives. The result is a representative sample of the highest Zn⁺ columns of gas within the DLA population. These Zn II-selected systems define the upper envelope of DLA metallicities near z = 1.2. They show a tight anti-correlation between N(H I) and [Zn/H], with the higher metallicity systems clearly exhibiting more depletion based on [Cr/Zn] values. The various metal-line measurements (Zn, Cr, Si, Fe, Mn) indicate evolved neutral gas with -0.9 ≲ [Zn/H] ≲ +0.4.